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XChangeSaveSet, XAddToSaveSet, XRemoveFromSaveSet

change a client's save set

int XChangeSaveSet(Display *display, Window w, int change_mode);

int XAddToSaveSet(Display *display, Window w);

int XRemoveFromSaveSet(Display *display, Window w);

Specifies the mode. You can pass SetModeInsert or SetModeDelete.
Specifies the connection to the X server.
Specifies the window that you want to add or delete from the client's save-set.

Depending on the specified mode, XChangeSaveSet either inserts or deletes the specified window from the client's save-set. The specified window must have been created by some other client, or a BadMatch error results.

XChangeSaveSet can generate BadMatch, BadValue, and BadWindow errors.

The XAddToSaveSet function adds the specified window to the client's save-set. The specified window must have been created by some other client, or a BadMatch error results.

XAddToSaveSet can generate BadMatch and BadWindow errors.

The XRemoveFromSaveSet function removes the specified window from the client's save-set. The specified window must have been created by some other client, or a BadMatch error results.

XRemoveFromSaveSet can generate BadMatch and BadWindow errors.

Some argument or pair of arguments has the correct type and range but fails to match in some other way required by the request.
Some numeric value falls outside the range of values accepted by the request. Unless a specific range is specified for an argument, the full range defined by the argument's type is accepted. Any argument defined as a set of alternatives can generate this error.
A value for a Window argument does not name a defined Window.

XReparentWindow(3)
Xlib - C Language X Interface